The invention relates to the optical transmission of information and, more particularly, to a method and apparatus for compensating for chromatic dispersion that accrues over optical fiber transmission systems.
The availability of high performance optical amplifiers such as the Erbium-Doped Fiber-Amplifier (EDFA) has renewed interest in the use of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels are combined onto a common path for transmission to a remote receiver. Typically, in a long-haul optical fiber system, the set of wavelength channels would be amplified simultaneously in an optical amplifier based repeater. The Erbium-Doped Fiber-Amplifier is particularly useful for this purpose because of its ability to amplify multiple wavelength channels without crosstalk penalty.
Typically, it is advantageous to operate long-haul transmission systems at high data rates per channel. For example, useful data rates include multiples of the Synchronous Digital Hierarchy (SDH) standard, i.e., 2.5 and 10 Gb/s. While both 2.5 and 10 Gb/s are available, in many applications it is desirable to operate at the highest standard bit rate possible. This lowers the cost and physical dimensions of the terminal equipment. For example, a transmission system with a capacity of 160 Gb/s would require 64 2.5 Gb/s transmitters/receiver pairs, whereas a system based on 10 Gb/s would only require 16 10 Gb/s pairs of terminal equipment. Unfortunately, as the transmission bit rates are increased, so do the transmission penalties associated with the transmission fiber""s chromatic dispersion and nonlinear index of refraction. It has been found both experimentally and by simulations that by using a combination of dispersion management techniques and distortion tolerant transmission formats, 10 Gb/s channels can be transmitted successfully over transoceanic distances over a limited range of wavelengths.
One of the more important parameters that limit the usable wavelength space is the so-called xe2x80x9cdispersion slopexe2x80x9d, or xe2x80x9chigher orderxe2x80x9d dispersion of the transmission fiber. The dispersion slope causes each WDM channel to experience a slightly different amount of dispersion. The efficacy of the dispersion mapping technique, reviewed by Forghieri et al., (Chapter 8 in Optical Fiber Telecommunications IIIA, Academic Press 1997) is limited because the amount of dispersion that occurs in a typical optical fiber depends on the operating wavelength that is employed. That is, only one given wavelength can operate at average zero dispersion. Accordingly, because of this characteristic of the dispersion slope, the various channels employed in a WDM system cannot all operate at the wavelength of average zero dispersion. This limitation can be overcome to a limited degree by using individual channel dispersion compensation at the receiver. However, since these systems are subject to nonlinear penalty, the ability to correct for the non-zero dispersion at the receiver terminal is limited. Thus, channels located far from the average zero dispersion wavelength can experience large amounts of distortion.
In accordance with the present invention, a method and apparatus is provided for managing dispersion in a WDM optical transmission systems so that transmission performance is improved using a so-called xe2x80x9csplit-bandxe2x80x9d dispersion management scheme. The split-band scheme is accomplished by providing select repeaters that can divide the WDM channels into two (or more) bands. All the WDM channels enter the first select repeater on a common fiber. The select repeaters are followed by sections of transmission cable having therein twice (or more) the usual number of optical fibers. These cable sections allow the bands to experience differing amounts of chromatic dispersion. The WDM channels are then re-combined onto a common fiber. Since in this arrangement differential dispersion is provided in an actual section of transmission cable rather than in a repeater, it is particularly efficient in terms of excess loss.
In one particular embodiment of the invention, a WDM optical communication system is provided that includes a transmitter and a receiver. An optical fiber transmission path couples the transmitter to the receiver. The transmission path includes at least one repeater having an optical amplifier located therein. A dispersion compensator is disposed at an intermediate point along the transmission path. The intermediate point is located outside of the repeater. The compensator includes a wavelength routing device for dividing a signal having a prescribed bandwidth into a plurality of distinct sub-bands. A plurality of output paths is provided for respectively receiving the plurality of distinct sub-bands. The dispersion compensator also includes a dispersion compensating optical element coupled to each of the output paths. Each dispersion compensating optical element substantially compensates for dispersion at a prescribed wavelength within the bandpass of its respective sub-band. A coupler recombining the distinct sub-bands and couples them back onto the optical fiber transmission path.